go/types: guard against check==nil in newNamed

When importing generic named types, it is possible for Checker.newNamed
to be called during type instantiation when the Checker is nil.

In this case we should be able to safely skip this delayed expansion.

Updates #45580

Change-Id: I75422100464d57eba24642c93e06e8b47d904fc3
Reviewed-on: https://go-review.googlesource.com/c/go/+/322974
Trust: Robert Findley <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>

Author: findleyr

src/go/types/type.go

@@ -661,11 +661,7 @@ func NewNamed(obj *TypeName, underlying Type, methods []*Func) *Named {
 	if _, ok := underlying.(*Named); ok {
 		panic("types.NewNamed: underlying type must not be *Named")
 	}
-	typ := &Named{obj: obj, orig: underlying, underlying: underlying, methods: methods}
-	if obj.typ == nil {
-		obj.typ = typ
-	}
-	return typ
+	return (*Checker)(nil).newNamed(obj, underlying, methods)
 }
 
 func (check *Checker) newNamed(obj *TypeName, underlying Type, methods []*Func) *Named {
@@ -681,13 +677,15 @@ func (check *Checker) newNamed(obj *TypeName, underlying Type, methods []*Func)
 	//
 	// TODO(rFindley): clean this up so that under is the only function mutating
 	//                 named types.
-	check.later(func() {
-		switch typ.under().(type) {
-		case *Named, *instance:
-			panic("internal error: unexpanded underlying type")
-		}
-		typ.check = nil
-	})
+	if check != nil {
+		check.later(func() {
+			switch typ.under().(type) {
+			case *Named, *instance:
+				panic("internal error: unexpanded underlying type")
+			}
+			typ.check = nil
+		})
+	}
 	return typ
 }